The rate at which energy is transferred is called power and the amount of energy that is usefully transferred is called efficiency. It is important to be able to calculate power and efficiency.
For which of these could the power be calculated?
An apple sitting still on a table
A shelf holding up a book
A toy car being pushed across the floor
What are the units for power?
Power is measured in watts (W)
Power is measured in amps (A)
Power is measured in joules (J)
Which equation about power is correct?
What is the definition of a watt?
100 joules transferred each minute
1 joule transferred each second
1 joule transferred each minute
Which of the following is a definition of power?
The amount of work done by a certain force
The energy transferred × time taken
The amount of work done ÷ time taken
A hair dryer transforms 10,000 J of energy in 4 s. What is its power?
A 60 W lamp is left on for 20 s. How much energy has been transformed?
In an electric kettle, what energy is a useful transfer?
The energy that heats the water
The energy that dissipates out of the kettle into the surroundings
The sound heard as the kettle boils
What is the efficiency percentage of Christmas lights that give out 10 J of light radiation for an electrical transfer of 200 J?
What percentage efficiency is an efficiency of 0.25?